One of the critical points for the use of a hairspring in a high-precision clockwork movement is the reliability of the attachment of the hairspring to the balance staff. This attachment is usually made by means of a collet, which was originally a small split cylinder designed to be driven onto the balance staff and pierced laterally in order to receive the inner end of the hairspring.
Therefore, Swiss patent No. CH 468662 describes, in particular in connection with its FIG. 3, a collet having four arms, a circular central opening, an elasticity split for the driving onto the balance staff and a recess for the balancing.
The subject of the European patent application published under No. EP 1 302 821 is a split collet with a circular central opening made in one block with a hairspring and comprising balancing recesses.
French patent application number FR 2 124 243 (U.S. Pat. No. 3,785,028) relates to circular sleeves for a hairspring that are designed to be driven onto the balance staff of a watch. These sleeves are manufactured from section pieces having an arm for the attachment of a hairspring and a regulating split diametrically opposite to this arm.
It therefore appears that many solutions for the driving of a collet onto a staff are known, and they are well suited to the usual materials such as steel which have a range of plastic deformation. Specifically, the diameter of the opening provided to receive the balance staff is smaller than the diameter of the balance staff so as to ensure a good hold for the staff after driving. This difference in diameter is usually absorbed at least in part by a plastic deformation of the material of the collet.
This means that these solutions are not very well suited to the collets or to the collet-hairspring assemblies made of a material such as silicon, quartz or diamond. Specifically, these materials, that can be machined by techniques such as deep reactive-ion etching (DRIE), have no range of plastic deformation, which means that they break when the stresses in the material exceed the elastic limit.
For example, Swiss patent No. 508 233 proposes a split collet of which the circular central opening comprises a second split of small-dimension elasticity. As can be seen in particular in FIG. 1, the two halves of this collet are extremely bulky which makes them very rigid and limits the amplitude of their elastic deformation. This makes the collet very fragile, in particular when it is made of a material such as silicon.
The subject of Swiss patent No. CH 252 387 is a collet consisting of two parts, a hollowed-out ring and a U-shaped elastic part placed transversely inside the hollowed-out ring. The two arms of the U are stressed by the hollowed-out ring and must deform in order to pinch the strip of the hairspring and accommodate the balance staff during driving. However, the elasticity at the connection of the arms of the U-shaped part is insufficient, which easily leads to the breakage of this part, in particular when it is made of a material such as silicon.
Moreover, the European patent application published under number EP 2 112 565 reveals microengineering parts having a central opening of which the contour is not circular but has a symmetry of rotation of 2π/3 about the axis passing through the center C of the central opening. FIG. 5 of this document shows in particular a microengineering part furnished with elastic arms.
The solutions described in this document are completely satisfactory for mechanical parts such as wheels. However, they have a number of drawbacks for producing collets that usually have less thickness than wheel plates, such as providing a low range of tolerance for the diameters of the staffs onto which the collets have to be driven.